Packaging containers of the single use disposable type for liquid foods are often produced from a packaging laminate based on paperboard or carton. One such commonly occurring packaging container is marketed under the trademark Tetra Brik Aseptic® and is principally employed for aseptic packaging of liquid foods such as milk, fruit juices etc, sold for long term ambient storage. The packaging material in this known packaging container is typically a laminate comprising a bulk core layer of paper or paperboard and outer, liquid-tight layers of thermoplastics. In order to render the packaging container gas-tight, in particular oxygen gas-tight, for example for the purpose of aseptic packaging and packaging of milk or fruit juice, the laminate in these packaging containers normally comprises at least one additional layer, most commonly an aluminium foil.
On the inside of the laminate, i.e. the side intended to face the filled food contents of a container produced from the laminate, there is an innermost layer, applied onto the aluminium foil, which innermost, inside layer may be composed of one or several part layers, comprising heat sealable adhesive polymers and/or polyolefins. Also on the outside of the core layer, there is an outermost heat sealable polymer layer.
The packaging containers are generally produced by means of modern high-speed packaging machines of the type that form, fill and seal packages from a web or from prefabricated blanks of packaging material. Packaging containers may thus be produced by reforming a web of the laminated packaging material into a tube by the two longitudinal edges of the web being united to each other in an overlap joint. The inner- and outermost heat sealable thermoplastic polymer layers within the overlap joint are welded together by applying heat. The tube is filled with the intended liquid food product and is thereafter divided into individual packages by repeated transversal seals of the tube at a predetermined distance from each other below the level of the contents in the tube. The packages are separated from the tube by incisions along the transversal seals and are given the desired geometric configuration, normally parallelepiped, by fold formation along prepared crease lines in the packaging material.
A layer of an aluminium foil in the packaging laminate provides gas barrier properties quite superior to most polymeric gas barrier materials. The conventional aluminium-foil based packaging laminate for liquid food aseptic packaging is the most cost-efficient packaging material, at its level of performance, available on the market today. Any other material to compete must be more cost-efficient regarding raw materials, have comparable food preserving properties and have a comparably low complexity in the converting into a finished packaging laminate.
Hitherto, there are hardly any aseptic paper- or paperboard-based packages for long-term ambient storage of the above described kind available on the market, from a cost-efficient, non-foil packaging laminate, as compared to aluminium-foil laminates, that have a reliable level of barrier properties and food preservation properties for more than 3 months. There are some polymer materials that provide good barrier properties, but they either have the wrong mechanical properties in the laminate or are difficult to process in the converting into thin layers in laminates, e.g. requiring expensive co extruded tie layers, or, they may, moreover, be considerably more expensive at feasible thickness than aluminium and are, therefore, not cost-efficient for packaging of e.g. milk or juice.
There is one type of polymer gas barrier layers that could be very cost-efficient, i.e. barrier polymers that are coated in the form of a dispersion or solution in a liquid or solvent, onto a substrate, and subsequently dried into thin barrier coatings. It is, however, very important that the dispersion or solution is homogeneous and stable, to result in an even coating with uniform barrier properties. Examples of suitable polymers for aqueous compositions are polyvinyl alcohols (PVOH), water-dispersible ethylene vinyl alcohols (EVOH) or polysaccharide-based water-dispersible or dissolvable polymers. Such dispersion coated or so called liquid film coated (LFC) layers may be made very thin, down to tenths of a gram per m2, and may provide high quality, homogenous layers, provided that the dispersion or solution is homogeneous and stable, i.e. well prepared and mixed. It has been known for many years that e.g. PVOH has excellent oxygen barrier properties under dry conditions. PVOH also provides very good odour barrier properties, i.e. capability to prevent odour substances from entering the packaging container from the surrounding environment, e.g. in a fridge or a storage room, which capability becomes important at long-term storage of packages. Furthermore, such liquid film coated polymer layers from water-dispersible or -dissolvable polymers often provide good internal adhesion to adjacent layers, which contributes to good integrity of the final packaging container. With package integrity is generally meant the package durability, i.e. the resistance to leakage of a packaging container. Such water dispersible barrier polymers have a major draw-back, however, in that they are generally sensitive to moisture and that the oxygen gas barrier properties deteriorate rapidly at high relative moisture content in the packaging laminate. Consequently, a thin dispersion coated layer of PVOH or EVOH or a similar polymer, may be suitable for packaging of dry products in a dry environment, but much less for packaging of liquids and wet products for long-term storage.
It has, therefore, previously been attempted to provide the moisture sensitive polymer layer with better initial oxygen barrier properties, as well as rendering it more moisture resistant, by modifying the polymer or including other substances in the polymer composition, i.a. by crosslinking the polymer. Such modifications and addition of substances, however, often make the process of liquid film coating more difficult to control and, importantly, more expensive. Such substances may also need careful screening in view of existing food safety legislations for food packaging. It has, for example, also been attempted to heat cure a dispersion coated PVOH layer in connection with the drying thereof, by heating it up to above 100° C. However, the heat exposure may damage the coated paperboard substrate and negatively influence the coating quality, for example by inducing defects, such as blisters and cracks in the oxygen barrier coating. Moreover, it has been realised that such attempts alone do not provide sufficient moisture resistance and robustness for keeping a sufficient level of oxygen barrier through the entire life of an aseptic package.
It was, thus, expected that in order to reach the required level of oxygen barrier properties in a final packaging container for aseptic, long-term storage, the liquid-film coatable gas barrier polymer binder, e.g. PVOH, would have to be improved by new means or at least by some of the expensive known modification methods, i.e. the addition of a crosslinking substance.
In addition, a packaging laminate for use in packaging containers for aseptic, long-term storage, needs to have water vapour barrier properties in itself, i.e. for the protection of the packaged food product.
It is important to understand that there is a difference between water vapour barrier and water vapour resistance of a layer. With water vapour or moisture resistance is meant the ability of a barrier layer to keep its barrier properties also when exposed to moisture, i.e. to withstand the negative influence of moisture on the properties of the polymer.
With water vapour barrier properties is meant the barrier against slowly migrating water vapour molecules through the material, i.e. not the ability to resist water or moisture in order to keep the properties of the material and not the immediate liquid barrier properties, which are preventing the material from getting wet in a short term perspective, i.e. immediately or almost immediately. As an example, heat sealable polyolefins, such as the preferred low density polyethylenes (LDPE's or LLDPE's), are liquid barriers and are suitable as outermost layers to protect the paperboard inside of a laminate against the filled liquid product or against wet conditions outside the package, such as at high humidity or chilled storage. Low density polyethylene has, however, comparably low water vapour barrier properties, i.e. actually very low capability at reasonable thickness, to withstand the long-term, slow migration of water vapour through the laminate during shipping and storage.
Water vapour barrier properties are important during long-term storage, also because they prevent moisture from a packaged liquid food product from escaping out of a packaging container, which could result in a lower content of liquid food product than expected in each packaging container, when finally opened by the consumer. Possibly, also the composition and the taste of the product could be altered by becoming more concentrated. Moreover, by preventing water vapour from migrating and escaping out of the packaged food product into the paper or paperboard layer, the packaging laminate will be able to keep its stiffness properties for a longer time. Thus, it is important that the packaging material also has sufficient water vapour barrier properties to be suitable for long-term aseptic packaging of liquid products.
The water dispersible or dissolvable barrier polymers suitable for liquid film coating or dispersion coating, generally have low resistance to water and moisture. They are easily loosing their oxygen barrier properties when exposed to moisture. They are not known to have water vapour resistance, unless they are crosslinked or modified in some way. When modified to obtain moisture resistance in order to maintain their gas barrier properties, normally they still do not obtain any notable water vapour barrier properties.
The conventional aluminium foil used today in commercial packaging containers for aseptic, liquid food, has both water vapour barrier properties and oxygen barrier properties. There are hardly any suitable, cost-efficient material alternatives providing both reliable oxygen barrier and water vapour barrier comparable to aluminium foil. Aluminium foil does, in fact, effectively prevent any molecules existing in the environment around the package or in the packaged product from migrating in any direction through the foil, as long as the aluminium foil layer is intact and undamaged.
There is a need, however, for a cost-efficient and robust, i.e. reliable also at moderate variations in manufacturing and handling conditions, non-aluminium foil based packaging material for aseptic, liquid food packaging, e.g. of milk or other beverage, which material provides sufficient total barrier properties in packaging containers for long-term aseptic storage, under ambient conditions. With the term “long-term storage” in connection with the present invention, is meant that the packaging container should be able to preserve the qualities of the packed food product, i.e. nutritional value, hygienic safety and taste, at ambient conditions for at least 3 months, preferably longer.